A point sensor/actuator (accelerometer or transducer) utilizing the piezoelectric effect reacts to or induces force, displacement, or acceleration at a point in a structure embodying the sensor/actuator. No a priori knowledge about the structure is needed to achieve the desired sensing or actuating. Discrete point sensors/actuators to detect and control the vibrations of flexible structures such as robot arms, satellite antennas and the like have been in existence for over thirty years. However, actuator/observer spillover due to residual (uncontrolled) vibration modes in conventional systems leads to instabilities in closed-loop control systems. The independent modal-space control method utilizing pre-filtering and modal filtering has been proposed to solve the spillover problem. In one prior art instance, a modal-filtering process generated signals from an array of discrete point sensors simultaneously and then fed the signals into a control loop circuit, but the large number of signals and subsequent amount of signal processing required resulted in significant phase delays in the control loop circuit, thereby rendering the system unreliable at best and inoperable at worst.
Conventional point-sensors operate at specific given points on a structure to collect motion signals, or they may operate as different structures themselves or in operate in different states to detect different specific motion/vibration signals. In cases where the measurement of motions or vibrations itself is not influenced by the device to be measured, the motion sensing and measurement is relatively simple. However, in practice, conventional point sensors are clearly affected by the characteristics of their own structure when making a motion detection and thus they are limited by an effective usable bandwidth (i.e., within which there is no self-effect).
Distributed surface electrode patterns and their directions of polarization can increase the effective use bandwidth of the sensors. Because electrodes can be distributed in space, in addition to being able to detect the total motion of a body, the distributed force on that body (the force on particular parts of the body) can also be measured. However, because distributed sensing requires a sensor pattern specific to the structure in question, the sensing system must be re-designed for each different application. Thus distributed sensing systems suffered from lack of general applicability. This is the principal reason to date why distributed sensor systems have not been as widely used as point sensor systems.
The piezoelectric effect is the phenomenon whereby certain materials, when subjected to a distorting force will produce an electrical polarization and a subsequent electromotive force. Conversely, when an electromotive force is applied to such a material, it will change its shape in response to that emf. Thus, piezoelectric materials can be used as motion sensors or actuators. However, conventional piezoelectric materials are typically crystalline or polycrystalline and therefore brittle and not easily conformable.